As a means for removing the moisture in air, moisture absorbers having much moisture absorptive capacity and high moisture absorptive rate such as lithium chloride, calcium chloride, magnesium chloride and phosphorus pentoxide have been used. However, since those moisture absorbers have deliquescence, there are disadvantages that they are liquefied after absorption of moisture, pollute others and corrode metal, etc., have a bad shape-holding property being hard to make into a molded product, have too strong moisture absorptive property being poor in moisture desorptive property and are not able to be used repeatedly.
In the case of moisture absorbers such as silica gel, zeolite, sodium sulfate, active alumina and active carbon, they are able to be used repeatedly because they have a moisture desorptive property in addition to a moisture absorptive property whereby a part of the aforementioned problems has been solved. However, there are disadvantages therein that they have little moisture absorptive capacity, require high temperature for regeneration, are crushed and pulverized by repeated absorption and desorption of moisture and have a big specific gravity resulting in a difficulty in mixing with resin.
Apart from the inorganic ones as such, there are some cases where water absorptive resins represented by polyacrylate type are used as moisture absorbers of an organic type. However, in the case of a water absorptive resin, although a water absorptive ability for liquid water is very good, its moisture absorptive ability for moisture which is gas is low and is inferior to the aforementioned inorganic moisture absorbers. The specific problems are that a retaining property for water is high being difficult for moisture desorption, that a moisture absorptive ability is high at a nearly saturated relative humidity while a moisture absorptive ability at a relative humidity lower than that is very low, that tackiness is noted as a result of absorption of moisture, that a big change in volume happens by absorption of moisture or by absorption of water and that, as taken up as a problem at present, moisture absorptive rate is very slow. They will be due to the following reasons that the phenomena of “water absorption” and “moisture absorption” show entirely different behavior although the object is the same “water” and that, in the conventional water absorptive resins, their chemical structures are designed for an object of absorption of water whereby a moisture absorptive property is not fully achieved. Such a difference between “water absorption” and “moisture absorption” will be illustrated later.
With regard to the aforementioned problems, technique where a target is a phenomenon of “moisture absorption” has been proposed in recent years. In Japanese Patent Laid-Open No. 05/132,858, a highly moisture absorptive and desorptive fiber consisting of organic polymer is proposed while, in Japanese Patent Laid-Open No. 08/225,610, moisture adsorptive and desorptive organic ultrafine particles are also proposed and they give one solution to a problem of highly saturated moisture absorptive property. However, a moisture absorptive property within short time is not improved and a slow moisture absorptive rate has been still pointed out as a problem.
With regard to the moisture absorptive rate, a method where polyacrylate and deliquescent inorganic salt are combined is proposed in the invention of Japanese Patent Laid-Open No. 05/105,704 for example. There is mentioned that, according to said method, a product having an improved moisture absorptive ability at low humidity and having a high moisture absorptive ability without dropping the liquid is prepared. However, with regard to the moisture absorptive rate, time required for moisture absorption is in an order of hours (in Examples, a significant increase in moisture absorptive amount is available during 10 hours) and such a method is not satisfactory for the use where moisture absorptive rate is required in an order of minutes or seconds such as in the case of a rotary dehumidifier.
In order to increase the rate for moisture absorption and desorption, it is usually effective for increasing the specific surface area and, with such an object, a method where porous thing is prepared, particle size is made small, etc. has been generally used. However, when particle size is made small in the conventional material having moisture absorptive and desorptive property, a method by means of a mechanical pulverization has been mostly used. In view of pulverizing ability or trapping efficiency in making into ultrafine powder however, products which are able to be prepared in an industrial scale are limited to those having an average primary particle size in an order of 1 μm. Although there are examples of 0.4 μm and 0.3 μm for the primary particle size in Japanese Patent Laid-Open Nos. 2000/17,101 and 10/237,126, respectively, they are unable tobe taken out in a form of primary particles as they are but they are only able to be taken out in particles where they are fused and aggregated whereby, as a result, the surface area of each of them does not effectively function. There is another problem that, since only aggregates having a particle size of several tens μm are prepared, it is not possible to manufacture thin film, etc.
Further, it is usual that, when a particle size becomes smaller, cohesive force among particles due to capillary pressure becomes larger whereby fusion among particles takes place and, as a result, surface of the particle is blocked and surface area as an aggregate becomes smaller. For example, when an emulsion having a primary particle size of not more than submicronmeter is applied and dried, a very dense film is obtained because of a very small particle size. When specific surface area of the film is measured, it is usually less than 1 m2/g. Thus, there is a problem that the advantage due to a surface area (calculated as not less than 50 m2/g) based on the particle size of not more than submicronmeter is blocked by fusion and is unable to be achieved whereby it does not contribute in improvement in moisture absorptive rate.
In Japanese Patent Laid-Open No. 2001/11,320, there is a description for a moisture absorptive and desorptive polymer having a carboxyl group of a potassium salt type and showing an excellent moisture absorptive and desorptive rate and, although some improvements in the moisture absorptive and desorptive rate are noted, its particle size is about several μm at the smallest and it has no sufficient ability for high-leveled ones which are demanded in practical use. In Japanese Patent Laid-Open No. 2003/231,863, there is also a description exemplifying the material where moisture absorptive and desorptive fine particles are added to paint but its main constituting material is paint and the moisture absorptive and desorptive fine particles are added only as an additive. Therefore, the moisture absorptive and desorptive fine particles are covered with the paint whereby their direct contact to moisture or air for their regeneration is disturbed and it is difficult to achieve a high moisture absorptive and desorptive property.